Porous membranes are widely used in various fields. Of these, a medical separation membrane, particularly a hemodialysis membrane, comprises natural polymer membranes made from cellulose and the like, and synthetic polymer membranes made from polysulfone, polymethyl methacrylate, ethylene-vinylalcohol copolymer, polyacrylonitrile, polyamide, polyallylether sulfone, polyester polymer alloy and the like.
Of the synthetic polymer membranes, a membrane using polyamide has been widely employed for medical use and the like, since it is superior in biocompatibility and free of bisphenol A, which is an endocrine disrupter feared to exert an adverse influence on human body, as a constitution component, and the like.
While medical separation membranes are always subjected to a sterilization treatment, autoclave sterilization (AC sterilization), which includes treatment with hot water, is widely used as one of the sterilization methods because of high safety and the like. However, since polyamide membranes generally show high water absorbability and they show degraded mechanical property in water absorption state, their size and structure vary particularly when treated with hot water, and AC sterilization has been difficult to apply.
JP-A-7-256067 describes a permselective membrane made from polyamide, which has a water content of not less than 8 wt % and heat of crystallization of not more than 30 mJ/mg. While this publication admits a disadvantage of polyamide membrane in the resistance to AC sterilization because higher water contents are associated with lower heat resistance when the membrane contains water, it concludes that AC sterilization of polyamide having a high water content is possible at 121° C., when the glass transition point of the dry resin is not less than 130° C. According to the study of the present inventors, however, it has been clarified that even a membrane produced using such polyamide fails to show sufficient dimensional stability and the like under AC sterilization conditions, and the AC sterilization markedly degrades membrane performance.
In view of the above-mentioned situation, the present invention aims at providing a porous membrane made from polyamide as a main material, which is useful as a medical separation membrane and the like, shows extremely small dimensional changes during a hot water treatment, and which particularly permits an AC sterilization treatment.